Field of the Invention
Higher olefins (e.g., acyclic alkenes of from 5 to 15+ carbon atoms) are produced commercially in large volumes by oligomerization over phosphoric acid catalysts using lower olefin feeds (for example, propylene, butenes and pentenes). Hydrocarbon Processing, vol. 47, page 170 (September 1968); Petroleum Refiner, vol. 36, no. 9, page 232 (1960). Prior art phosphoric acid catalysts which have been developed have employed a wide variety of catalyst supports, e.g. those disclosed in U.S. Pat. Nos. 2,586,852 and 2,713,560 (each to Morrell), phosphoric acid on such supports as activated carbon, silica gel, diatomaceous earth, kiesulguhr, infusorial earth, bentonite, montmorillonite and similar adsorbent substances. This patentee also discloses that catalysts can be formed by baking at elevated temperature a mixture of kaolin and phosphoric acids (e.g., orthophosphoric, pyrophosphoric, metaphosphoric, and tetraphosphoric acids), and that the porosity of such kaolin catalysts can be improved by adding from 1 to 10% (based on the kaolin) by weight of calcium and/or magnesium oxides, or the corresponding carbonates, to the kaolin before mixing in the phosphoric acid. It is noted in these patents that mixtures of calcium oxide and magnesium oxide are preferred, because the former produces a more porous product but one which is softer and takes up water quite readily, whereas these features are corrected by the magnesium.
Equipment fouling in such higher olefins plants has been a serious problem since the 1930's. Heretofore, equipment sparing was thought to be the only means to effectively combat this problem. This has necessitated maintaining multiple trains of vital equipment to enable continued plant operation while simultaneously cleaning fouled equipment that have been taken out of service. The debits associated with fouling for each higher olefins plant range from thousands to millions of dollars per year depending on several factors, among these, availability of spare heat exchangers, distillation towers, maintenance costs, frequency of re-tubing of reactors and exchangers, etc.
Past attempts to minimize the degree of fouling (e.g., using neutralization, filtration and percolation) have met little success. Also, attempts have been made to introduce organic anti-foulants into process streams, but these have not created satisfactory solutions to the fouling in downstream equipment.
Canadian Pat. No. 507,337 is directed to a method of inhibiting metal corrosion in a polymerization reactor employing a bulk liquid phosphoric acid catalyst wherein phosphoric acid esters (reaction products of phosphoric acid and the feed olefins and/or the olefinic oligomerization products) are maintained in the reaction mixture below about 0.08 mol of such esters per mol of free phosphoric acid. In the process, the olefin feed is dried (e.g., with silica or alumina), heated and then introduced into the reactor. The reactor's liquid effluent is withdrawn and phase separated to form a lower phosphoric acid phase (which is recycled to the reactor) and an upper hydrocarbon phase, which is treated for product recovery. A portion of the separated phosphoric acid phase is heated in a separate vessel to thermally decompose the phosphoric acid esters. However, this method can result in fouling of the heat treatment vessel and only addresses corrosion problems in the reactor. No provision is made for avoiding of fouling in downstream hydrocarbon processing equipment. Moreover, thermal treatment of the separated hydrocarbon phase could result in undesired loss of hydrocarbon product, due to increased polymerizations catalyzed by the phosphoric acid thermal decomposition by-products.